Cladding of metals



July 5, 1966 w. RESNICK CLADDING OF METALS Filed April 10, 1965 wmIoZ mN w Zwwmum GRIT % REDUCTION INVENTOR. WALL ACE RESNICK his ATTORNEY United States Patent 3,258,839 CLADDING 0F METALS Wallace Resnick, Irvington, N.J., assignor to Jones &

Laughlin Steel Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed Apr. 10, 1963, Ser. No. 272,031 6 Claims. (Cl. 29-4701) This invention relates to the cladding of harder metal with a softer metal. It is more particularly concerned .with a process of cladding a ferrous metal with a softer metal foil.

It is desirable for many purposes to coat a relatively hard, strong, malleable material such as steel with a softer non-ferrous metal. Tin and zinc are easily coated on steel by hot-dipping or by electroplating. Aluminum is less easily coated by those processes. When a thin coating of the non-ferrous metal is desired, neither hot-dipping nor electroplating processes are entirely satisfactory because the coating produced by both tends to be porous or marred by pinholes.

A metal can be coated witha different metal by cladding, that is, bringing together the metals in the form of sheets or the like and rolling or otherwise cold-reducing them together. The cladding process has the advantage that each metal can beinspected in advance for pinholes and like defects; A thin coat or foil of a softer material can be coated ,upon a harder metal by cladding, with the assurance that the coating will be as free from pores and pinholes as is the foil. In spite of this advantage, cladding processes are not as widely used as hot-dipping or electroplating processes for the coating of a metal with another metal.

The problems involved in cladding one metal with another differ somewhat, depending upon the particular metals concerned. In general, the bond between base metal and coating metal may be achieved either thermally or mechanically, or by a combination of both effects. Where steel is the base metal and aluminum is the coating metal, investigators have found it very difficult to bring about satisfactory bonding by heat treatment of the composite article. The temperature which must be reached for thermal diffusion between the constituents of the article are so close to those at which brittle ironaluminum alloys form that adequate temperature control of the process is difficult to insure. If the composite article is not heated, the bond must be brought about by rolling pressure only, and the principal problem in such a process is that of surface preparation of the steel base. I describe my invention hereinafter with reference to the cladding of a low-carbon, flat-rolled steel base with a foil of aluminum by rolling without any heating other than that arising from the rolling itself. The term flatrolled steel comprehends steel both in the form of separate sheets and continuous strip. It will be understood, however, that my invention applies as well to the cladding of other relatively harder metals with relatively softer metals.

It is an object of my invention to provide a process of cold-cladding a harder metal with a softer metal which results in a stronger bond between the metals that has heretofore been obtained. It is another object to provide a process which can be more economically practiced than heretofore. It is yet another object to provide a process resulting in a strong bond but which requires less surface preparation of the harder metal than heretofore. Other objects of my invention will appear in the description thereof which follows.

Experience has shown that the best bond between steel and aluminum is obtained when the steel surface has first been grit-blasted with ferrous metal grit. The grit roughens or abrades the steel surface in such a way that the softer aluminum is rolled into a locking engagement with the base metal. It was thought heretofore that abrading the steel surface by other materials, or roughening it in other ways, such as by pickling or by wire brushing, does not bring about as strong a bond with the softer coating metal. No way of directly measuring the bondforming properties of a roughened steel surface has yet been devised to my knowledge. Several steel sheets roughened by grit-blasting may have comparable roughness as measured by a profilometer, but they may develop quite different bond strengths when they are clad with aluminum. The bond-forming properties of grit-blasting the steel appear to depend upon the properties of the steel grit, in a way to be described.

The efficacy of grit-blasting in the cladding process is a function of the size and sharpness of the grit and as the grit is re-used it breaks down and its sharp corners become rounded off. It is therefore necessary from time to time to screen the grit and remove the fraction below a certain size. The fraction removed must, of course, be replaced with fresh grit and the cost of this replacement is a very significant factor in a cladding process. Although the grit is constantly changing in its physical characteristics as it is used over and over again, it can be considered as a stable aggregate if, when it is periodically screened, the weight of the fraction discarded each time remains constant. I find that fresh grit stabilizes in this manner after relatively few cycles of use, on the order of 50, and remains stable for a very considerable number of cycles, a thousand or more. I find that the particle size distribution of the stabilized grit remains constant. The grit can be stabilized on any screen of a mesh size smaller than the original size of the grit, but, obviously, it is most economical to stabilize the grit on the smallest screen that will permit the production of the desired surface on the steel. The screen size on which the grit is stabilized is therefore inversely proportional to the bonding properties of the abraded steel.

In the foregoing paragraphs I have said nothing about the cold reduction required to bond the aluminum to the steel, or the ways of measuring that bond. Generally speaking, the greater the reduction effected, the better the bond that results therefrom. I know of no generally accepted test for measuring the bond between the cladding metal and the base metal of clad material. I prefer to measure the bond between aluminum and steel by cutting a flat tensile test specimen from the clad material and pulling it to fracture in a tensile testing machine. I then examine the adhesion of the aluminum to the steel at the fracture. If the specimen exhibits no spalling, or only slight spalling, I consider the bond satisfactory.

I have found that if a soft cladding metal such as aluminum is embossed before it is cold reduced with a harder metal such as steel, the latter requires much less critical surface preparation to produce a strong bond between aluminum and steel.

In an embodiment of my invention presently preferred by me, I employ as base metal cold-rolled unalloyed lowcarbon steel sheet of A181 1010 grade. For the coating material I employ aluminum foil .001" thick, embossed in a pattern of peaks and pits, a peak on one side being a pit or depression on the other. These peaks and pits are spaced about Ms" apart over the surface of the foil. The overall thickness of the embossed foil is about .006". The steel and embossed aluminum are brought together and passed through a rolling mill having smooth rolls about three (3) inches in diameter. I reduce the thickness of the composite material at least 5% and not more than about 45% The attached figure, to which reference is now made, illustrates the relation between surface preparation of the base metal and degree of reduction of the composite material for the process of my invention, as well as for the process of cladding steel with unembossed aluminum foil of the same thickness. The abscissa is the size of the mesh on which the grit was stabilized; the ordinate is the percentage reduction of cross-sectional area of the composite material which results in only slight spalling ofv a test piece pulled in tension as has been described. The upper, or solid line graph, is for material produced by the process of my invention while the lower, or broken line graph, is for material made up with flat or unembossed foil. If the composite material is reduced about 40% or more, an excellent bond is obtained between the embossed foil and steel previously pickled in the conventional way with a'sulphuric acid solution and degreased. If lighter reductions are given the material, the steel must be grit blasted to obtain a good bond, but the grit blasting can be carried out with grit stabilized on a substantially smaller screen than is required if the steel is coldreduced the same amount with unembossed foil.

For a given cold-reduction, embossed foil can be bonded to steel grit blasted with much finer grit than is required if flat foil is used. The use of finer grit is, of course, much more economical. It will be noted that grit stabilized on a screen of about .01" mesh was just as effective as new grit in preparing the surface of steel for cladding with embossed aluminum.

I claim:

1. The process of cold-cladding a harder metal with a softer metal foil comprising roughening the harder metal, bringing the roughened harder metal together with the foil embossed in a pattern of pits and peaks, so that the overall thickness of the foil is several times its thickness in the unembossed state, and cold-reducing together the roughened harder metal and the embossed foil.

2. The process of claim 1 in which the distance between an embossed peak and an adjoining pit is not more than about A;

3. The process of claim 1 in which the harder metal is roughened by grit-blasting.

4. The process of claim 1 in which the harder metal is flat-rolled steel and the softer metal is aluminum foil about .001" in thickness before embossing and about .006" overall thickness after embossing.

5. The process of claim 1 in which the harder meta is steel roughened by grit-blasting with ferrous metal grit stabilized on a screen, and the amount of cold-reduction is related to the screen size in accordance with the solid line curve of the figure.

6. The process of cold-cladding a harder metal with a softer metal foil comprising roughening the harder metal, embossing the foil in a pattern of pits and peaks so that its overall thickness is several times its thickness in the unembossed state, bringing together the roughened harder metal and the embossed foil and cold-reducing them together.

References Cited by the Examiner UNITED STATES PATENTS 1,667,787 5/1928 Jaeger et a1. 29470.1 X 2,443,870 6/1948 Reynolds 29-470.1

JGHN F. CAMPBELL, Primary Examiner. 

1. THE PROCESS OF COLD-CLADDING A HARDER METAL WITH A SOFTER METAL FOIL COMPRISING ROUGHENING THE HARDER METAL, BRINGING THE ROUGHENED HARDER METAL TOGETHER WITH THE FOIL EMBOSSED IN A PATTERN OF PITS AND PEAKS, SO THAT THE OVERALL THICKNESS OF THE FOIL IS SEVERAL TIMES ITS THICKNESS IN THE UNEMBOSSED STATE, AND COLD-REDUCING TOGETHER THE ROUGHENED HARDER METAL AND THE EMBOSSED FOIL. 